Catalytic Transformation of Levulinic Acid to 2‑Methyltetrahydrofuran Using Ruthenium–<i>N</i>‑Triphos Complexes

A series of pre- or in situ-formed ruthenium complexes were assessed for the stepwise catalytic hydrogenation of levulinic acid (LA) to 2-methyltetrahydrofuran (2-MTHF) via γ-valerolactone (γVL) and 1,4-pentanediol (1,4-PDO). Two different catalytic systems based on the branched triphosphine ligands Triphos (CH₃C­(CH₂PPh₂)₃) and <i>N</i>-triphos (N­(CH₂PPh₂)₃) were investigated. The most active catalyst was the preformed ruthenium species [RuH₂(PPh₃)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>5</b>), which gave near quantitative conversion of LA to 1,4-PDO when no acidic additives were present, and 87% 2-MTHF when used in conjunction with HN­(Tf)₂. Various acidic additives were assessed to promote the final transformation of 1,4-PDO to 2-MTHF; however, only HN­(Tf)₂ was found to be effective, and NH₄PF₆ and <i>para</i>-toluenesulfonic acid (<i>p</i>-TsOH) were found to be detrimental. Mechanistic investigations were carried out to explain the observed catalytic trends and importantly showed that PPh₃ dissociation from <b>5</b> resulted in its improved catalytic reactivity. The presence of acidic additives removes catalytically necessary hydride ligands and may also compete with the substrate for binding to the catalytic metal center, explaining why only an acid with a noncoordinating conjugate base was effective. Crystals suitable for X-ray diffraction experiments were grown for two complexes: [Ru­(NCMe)₃{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>14</b>) and [Ru₂(μ-Cl)₃{N­(CH₂PPh₂)₃-κ³<i>P</i>}₂]­[BPh₄] (<b>16</b>)

Insights into the Mechanism of Carbon Dioxide and Propylene Oxide Ring-Opening Copolymerization Using a Co(III)/K(I) Heterodinuclear Catalyst

A combined computational and experimental investigation into the catalytic cycle of carbon dioxide and propylene oxide ring-opening copolymerization is presented using a Co(III)K(I) heterodinuclear complex (Deacy, A. C.Co(III)/Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-Opening Copolymerization of CO2 and Propylene Oxide. J. Am. Chem. Soc.2020, 142(45), 19150−19160). The complex is a rare example of a dinuclear catalyst, which is active for the copolymerization of CO2 and propylene oxide, a large-scale commercial product. Understanding the mechanisms for both product and byproduct formation is essential for rational catalyst improvements, but there are very few other mechanistic studies using these monomers. The investigation suggests that cobalt serves both to activate propylene oxide and to stabilize the catalytic intermediates, while potassium provides a transient carbonate nucleophile that ring-opens the activated propylene oxide. Density functional theory (DFT) calculations indicate that reverse roles for the metals have inaccessibly high energy barriers and are unlikely to occur under experimental conditions. The rate-determining step is calculated as the ring opening of the propylene oxide (ΔGcalc† = +22.2 kcal mol–1); consistent with experimental measurements (ΔGexp† = +22.1 kcal mol–1, 50 °C). The calculated barrier to the selectivity limiting step, i.e., backbiting from the alkoxide intermediate to form propylene carbonate (ΔGcalc† = +21.4 kcal mol–1), is competitive with the barrier to epoxide ring opening (ΔGcalc† = +22.2 kcal mol–1) implicating an equilibrium between alkoxide and carbonate intermediates. This idea is tested experimentally and is controlled by carbon dioxide pressure or temperature to moderate selectivity. The catalytic mechanism, supported by theoretical and experimental investigations, should help to guide future catalyst design and optimization

Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of N‑Centered Triphosphine Ligands

The reactivity of the novel tridentate phosphine ligand N­(CH₂PCyp₂)₃ (N-triphos^Cyp, <b>2</b>; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N­(CH₂PPh₂)₃ (N-triphos^Ph, <b>1</b>). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands <b>1</b> and <b>2</b> with the precursors [Ru­(COD)­(methylallyl)₂] (COD = 1,5-cycloocatadiene) and [RuH₂(PPh₃)₄] gave the tridentate coordination complexes [Ru­(tmm)­{N­(CH₂PR₂)₃-κ³<i>P</i>}] (R = Ph (<b>3</b>), Cyp (<b>4</b>); tmm = trimethylenemethane) and [RuH₂(PPh₃)­{N­(CH₂PR₂)₃-κ³<i>P</i>}] (R = Ph (<b>5</b>), Cyp (<b>6</b>)), respectively. Ligands <b>1</b> and <b>2</b> displayed different reactivities with [Ru₃(CO)₁₂]. Ligand <b>1</b> gave the tridentate dicarbonyl complex [Ru­(CO)₂{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>7</b>), while <b>2</b> gave the bidentate, tricarbonyl [Ru­(CO)₃{N­(CH₂PCyp₂)₃-κ²<i>P</i>}] (<b>8</b>). This was attributed to the greater electron-donating characteristics of <b>2</b>, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex <b>7</b> was activated via oxidation using AgOTf and O₂, giving the Ru­(II) complexes [Ru­(CO)₂(OTf)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}]­(OTf) (<b>9</b>) and [Ru­(CO₃)­(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>11</b>), respectively. Hydrogenation of these complexes under hydrogen pressures of 3–15 bar gave the monohydride and dihydride complexes [RuH­(CO)₂{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>10</b>) and [RuH₂(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>12</b>), respectively. Complex <b>12</b> was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH₄PF₆ in acetonitrile, forming [RuH­(CO)­(MeCN)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}]­(PF₆) (<b>13</b>), which reacted cleanly with LA to form [Ru­(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}­{CH₃CO­(CH₂)₂CO₂H-κ²<i>O</i>}]­(PF₆) (<b>14</b>). Complexes <b>3</b>, <b>5</b>, <b>7</b>, <b>8</b>, <b>11</b>, and <b>12</b> were characterized by single-crystal X-ray crystallography